1. Field of the Invention
This invention relates to a presensitized lithoprinting plate which needs no fountain solution. More particularly, the invention is concerned with a presensitized lithoprinting plate (for brevity it will hereinafter be referred to as PS plate) which needs no fountain solution, characterized by a photosensitive coating layer comprising a mixture of a photosensitive material and a silicone rubber.
2. Description of the Prior Art
A lithoprinting plate which needs no fountain solution has as its objects a simplification of the traditional off-set printing technique. In conventional lithography in which off-set processes are predominant, a printing plate is prepared by exposing a PS plate through a negative or positive image and treating the so exposed PS plate with an appropriate treating solution to form imagewise oleophilic areas and hydrophilic areas. On printing, a fountain solution is applied to the hydrophilic areas, i.e. the non-image areas, and, thereafter, an ink is applied to the oleophilic areas, i.e. the image areas. In this process, the application of the fountain solution is very troublesome and this is a significant drawback of the process. One method of eliminating this drawback is a lithoprinting process which does not require a fountain solution. In this process, practical printing is attained by using a printing plate having a layer of a highly ink-repellent silicone rubber and supplying an oily ink without application of any fountain solution. Some techniques for the manufacture of PS plates in this manner have been disclosed in Japanese Pat. Specification published for public inspection Nos. 23042/1969, 2361/1972 and 16044/1971 and U.S. Pat. Nos. 3,511,178 and 3,632,375.
In the processes disclosed in the Japanese Patent Specification published for public inspection No. 16044/1971 and U.S. Pat. No. 3,511,178, a PS plate having a layer of a silicone rubber on a layer of a photosensitive material applied to a support is used. The PS plate is exposed through a negative image and treated with a processing solution to remove the photosensitive layer and the silicone rubber layer in the exposed areas, i.e. the image areas. On the other hand, in the unexposed areas, i.e. the non-image areas, the silicone rubber remains unchanged. When an ink is applied to the printing plate thus obtained, the ink adheres to the image areas only. The PS plate of this type which does not require a fountain solution has a layer of a silicone rubber over a photosensitive layer, so that there is a problem of adhesion between the two layers. An intermediate layer containing a diazo compound between the two layers for enhancing the adhesion is needed and, consequently, the plate has a threelayered structure. Since a condensate of p-diazo-diphenylamine and formaldehyde employed as the intermediate layer, i.e. the adhesion-improving layer, is hydrophilic it is difficult to apply the condensate uniformly to a photosensitive layer comprising of oleophilic binder. In addition, merely interposing te condensate as an intermediate layer between the silicone rubber layer and photosensitive layer does not enhance the adhesion and it is necessary to heat, after application thereover of a silicone rubber layer, at 90.degree.C for 5 to 10 minutes in an oven to cure the silicone rubber and then at 120.degree.C for 10 minutes to thermally decompose the p-diazodiphenylamine-formaldehyde condensate to obtain the desired effect. The manufacture of the printing plate using this process is greatly restricted on account of these shortcomings.
In the processes disclosed in Japanese Pat. Specification published for public inspection Nos. 23042/1969 and 2361/1972, a film of a water-soluble polymer is formed on a strippable temporary support and a diazo photosensitive material is applied thereto. To a support separately provided, a silicone rubber is applied and the photosensitive layer is then forced to the surface of the uncured silicone rubber. After curing of the silicone rubber the temporary support is stripped off. The PS plate thus obtained is exposed through a negative image and processed to remove the photosensitive layer in the unexposed areas, i.e. the non-image areas. Thus, the surface of the silicone rubber of the under layer is exposed while the irradiated image areas remain unchanged. When an ink is applied to the plate thus obtained, the ink adheres to the image areas only. In this process, it is necessary to employ a temporary support to solve a difficult problem when a hydrophilic diazo photosensitive material is applied to a water-and oil-repellent silicone rubber. Thus, the manufacture of the printing plate in this process is even more restricted by these two-fold drawbacks, i.e., use of a temporary support in addition to the multi-layered structure of the plate.
In the process disclosed in U.S. Pat. No. 3,632,375, a silicone rubber is applied to a support and, while the silicone rubber is still in an uncured state, a high molecular weight adhesion improving material is spread thereover. An aqueous solution of a diazo photosensitive compound containing an emulsion of a film-forming material is spread thereover and the silicone rubber is cured. This process also involves the troublesome technique of applying a finely divided water-soluble polymer as an adhesion improver to attain firm bonding between the silicone rubber and the photosensitive material. It is evident that the manufacture of the plate in this process will be much more restricted by this troublesome step.
The problems in the prior art can be summarized as follows:
1. The plate is of a multi-layered structure wherein a photosensitive material and a silicone rubber form two distinctly separate layers.
2. It is important in the manufacture of the plate to attain strong bonding between the two layers.
3. In case of PS plates in which the uppermost layer is a silicone rubber layer, the silicone rubber layer is limited in its thickness to permit penetration of a developing solution.
4. Where a photosensitive layer is formed on a silicone rubber layer, it is necessary to use an extra, such as a temporary, support and, in addition, a sufficient improvement in adhesion between the silicone rubber layer and photosensitive layer can not be expected.